1. Field of the Invention
This invention relates to single stage electronic energy converters operated from alternating power line and capable of supplying, at the output, a load such as gas discharge lamp.
2. Description of Prior Art
The electronic energy converters, or as sometimes called "switching power supplies" need to operate directly from the alternating power line. Electric utility companies are setting requirements for specific groups of electricity-powered appliances in regards to power quality drawn by these appliances.
The electronic ballast, as one of the appliances, is used in large quantities in lighting fixtures. In general, to meet the industry requirements in regards to power quality, an electronic ballasts has to meet two fundamental requirements: (i) draw power from the power line with power factor (PF) of at least 0.9, (ii)draw current from the power line with total harmonic distortion (THD) of less than 20 percent.
The electronic ballast has to meet other requirements related to compatibility with a lamp-load. It shall provide 1amp current crest factor of less than 1.7, where the "crest factor" is equal to a peak magnitude of the lamp current divided by its effective (RMS) value. This is related to maximum allowable modulation of the lamp current magnitude, which is responsible for light flicker and poor lamp efficacy expressed in lumens of light produced from each watt of power consumed. It is desirable to have constant power to be delivered to the lamp load over the entire cycle of the voltage supplied by the power line.
In order to convert the low frequency power line alternating voltage (120V/60 Hz or 220V/50 Hz) to high frequency (typically from 10 kHz to 100 kHz) alternating voltage or current source, one has to rectify the signal from the power line to a DC voltage which later is converted, by switching transistors, to the high frequency source.
Conventional off-line rectifiers have a capacitive smoothing filter located beyond a diode rectifier circuit. This smoothing capacitor causes harmonic distortion of the current waveforms during periods in which the rectified output is higher than the voltage over the smoothing capacitor, and during which time the capacitor charges up. This charging time, or conduction angle, is very small if large capacitor is used, and all the required charge has to be loaded into the capacitor in a short period of time. This results in a large current output from the rectified power line source. These current spikes increase the harmonic content of the power supply, and when large number of ballasts are operated from the power line, this increased harmonic distortion causes a poor power factor in the supply. This situation is not accepted upon by electricity supply authorities and causing interference with other electrical equipment.
Techniques for improving power factor include passive waveform shaping methods. One of them is described in U.S. Pat. No. 5,150,013 issued to BOBEL. This method requires an inductor to operate in resonant mode with a capacitor, and the resonant frequency is approximately 180 Hz when power line frequency is 60 Hz. Is is inexpensive and reliable method. However, the inductor must be large in size.
It is known to use a storage conversion principle, whereby an inductor is controlled at high frequency in order to allow charging of the smoothing capacitor over wide conduction angle. The system, however, requires a control circuit for the storage converter, known also as "boost converter", in order to regulate the discharge of energy from the storage inductor. Such use of the storage conversion principle requires additional noise filtering, because large amount of noise is being generated by switching devices. The circuit is very complex and expensive to produce. Furthermore, the second stage converter is necessary to convert the DC voltage source to the high frequency alternating voltage or current source. This type circuit is described in U.S. Pat. No. 5,049,790 issued to Herfurth.
It is highly desirable to have simple and low cost single stage electronic energy converter. Such a circuit shall have low parts count and cost, it shall be adaptable to all power line voltages and lamps kinds, it shall be easy manufacturable in large quantities with great repeatability as required by industry quality standards, it shall meet the power quality standards and draw power from the power line with near-sinusoidal current waveform and provide near-constant power to the lamp over the entire cycle of power line voltage waveform.